The invention relates to a method for recovering metallic chromium from slags which contain chromium oxide.
In the conventional converter processes, such as the AOD, MRP, AOD-L, MRP-L, CLU, ASM, Conarc stainless steel or vacuum processes, such as VOD, SS-VOD, RH, RH with lanze, the basic decarburization reaction is carried out in a multiple substance system, the reduction of the chromium oxide using the carbon representing the basic mechanism.
After the primary oxidation of the chromium, which occurs in a high concentration in stainless steels, the carbon which is dissolved in the melt causes the chromium to be reduced at the bubbling surface and in what is known as the impingement area. The product of the reduction, namely the metallic chromium, is returned to the melt, and the carbon monoxide, after diffusion into the bubbles, is discharged into the gas atmosphere above the melt.
The chemical sequence of reactions is as follows:
{O2}=2[O]Dissociationxe2x80x83xe2x80x83(1)
[C]+[O]={CO}Direct decarburizationxe2x80x83xe2x80x83(2)
2[Cr]+3[O]=(Cr2O3)Chromium oxidationxe2x80x83xe2x80x83(3)
The reactions proceed under the following thermodynamic equilibrium                               K          ⁡                      (            T            )                          =                                            a              Cr              2                        ⁢                          p              CO              3                                                          a              C              3                        ⁢                          a                                                Cr                  2                                ⁢                                  O                  3                                                                                        (        4        )            
where                               log          ⁢                      xe2x80x83                    ⁢                      K            ⁡                          (              T              )                                      =                              40536            T                    +          25.63          +          p                                    (        5        )            
where p=a parameter.
On account of the incomplete reduction of chromium during the decarburization, a partial quantity of the chromium oxide passes into the slag in the form of various spinels. The reduction effect becomes increasingly weak as the decarburization progresses, since the amount of the reducing element carbon decreases over the course of time. The economic viability of the process is based on recovering a chromium which is-bonded in this way. For this purpose, the reduction of the slag using high-affinity silicon in the form of FeSi is normally carried out at the end of the decarburization and of the oxygen blowing operation.
An object of the present invention is to provide a more simple and therefore more economical method for recovering metallic chromium from slags.
This object is achieved by tapping, i.e., drawing off, the slag which is produced at the end of a blowing or treatment operation in a converter or a vacuum installation in unreduced form.
This slag is then charged into an electric furnace, which is also charged with a standard charge of scrap and, if appropriate, dust residues.
Carbon and, if appropriate, silicon are additionally added
During the melting process, the chromium oxide contained in the added slag is directly reduced to metallic chromium by the carbon and the silicon.
The conventional slag reduction step is omitted in the method according to the present invention. The slag, which is highly saturated with chromium oxide and maganese iron oxide and is present, for example, after an oxygen blowing operation, is removed from the metal, i.e., is tapped or drawn off from the ladle, in unreduced form and is charged into the upstream electric furnace. The slag is directly reduced by an addition of carbon and silicon from the scrap which is usually present in the electric furnace, if appropriate together with dust residues. In this way, the chromium oxide is reduced in the electric furnace instead of in one of the downstream installations, and the metallic chromium is thereby recovered.
The reduction reaction proceeds in accordance with the following equation:
2(Cr2O3)+3[Si]=4[Cr]+3(SiO2)xe2x80x83xe2x80x83(6)
In the electric furnace, direct reduction of the chromium oxide by means of the carbon and defined quantities of the silicon takes place under the atmospheric conditions, this reaction proceeding as follows:
(Cr2O3)+3[C]=2[Cr]+3{CO}xe2x80x83xe2x80x83(7)
or with silicon
2(Cr2O3)+3[Si]=4[Cr]+3(SiO2)xe2x80x83xe2x80x83(8)
The flow of the slag back into the electric furnace can take place immediately after each treatment or as a cumulative quantity from a number of different treatments.
Depending on the overall technology, this operation can be optimized in terms of cost and energy. It is of no importance for the method whether the slag is in a solid, liquid or intermediate state. This only affects the melting time required in the electric furnace.
Overall, this procedure results in the following economic and technological advantages:
shortening of the overall treatment time of the melt 15-20 min, depending on the type of technology
reduction in the consumption of FeSi
reduction in the consumption of slag-forming agents high yields of metal
increased service life of the refractory material
increased service life of the nozzles and plugs
improved energy balance of the converter or the vacuum installation
improved purity of the metal
elimination or considerable reduction of protective slag zones in converters and ladles